As they say that the moon keeps falling to the Earth, does the Earth too keep falling to the Sun? Never reaching it. Just locked in this forever fall, translated as revolution for us?
Yes. Exactly that.
Newton got there with a thought experiment about a cannon on a very high mountain. Fire the ball weakly and it arcs down and lands. Fire it harder and it lands further away. Fire it hard enough and the ground curves away beneath it as fast as it drops — so it keeps falling and never arrives. That's an orbit. Nothing else is happening.
The numbers are oddly small. The Sun's pull at our distance gives about 0.006 m/s², so in any given second the Earth falls roughly 3 millimetres toward it. In that same second we travel about 30 kilometres sideways, and over that distance the Earth's curved path drops away by — 3 millimetres. The fall and the escape cancel with a precision that feels like it ought to be a coincidence and isn't. The Moon does the same over us, falling about 1.4 mm per second.
Two footnotes to the picture, both of which I think improve it.
The first is that general relativity changes what "falling" means. In Einstein's account there's no force pulling on us at all; the Sun's mass curves spacetime, and the Earth is simply going straight, in the only sense of straight available to it. A body in free fall feels nothing — no strain, no force, weightless. What you feel right now is the floor pushing up on you, interrupting your fall. So falling isn't the strenuous state. Standing still is. The Earth's orbit is what happens when nothing at all is being done to it.
The second is that "forever" is generous. The orbit isn't a circle, so the fall has a rhythm: we genuinely close on the Sun from July to early January and genuinely climb away again the other half. And the arrangement leaks. The Moon isn't holding station — tides are pushing it outward about 3.8 cm a year, so its fall is very slowly loosening. The Sun is shedding mass and our orbit is creeping wider, until in a billion years or so the expanding Sun makes the question moot. The lock is real but it's a long unwinding, not a permanence.
Worth adding that it doesn't stop with us. The Sun is falling around the galactic centre, about 230 million years to the circuit, and the galaxy is falling toward the Virgo cluster. Nothing in the sky is at rest. It's falls inside falls, all the way up.
And where does the Virgo cluster fall?
Toward Shapley, by way of the Great Attractor — though the honest answer is that at this scale the chain starts to break down, which is the more interesting part.
The immediate picture: our Local Group falls toward Virgo at a few hundred kilometres per second. But Virgo itself, and everything around it, streams toward a region in Centaurus and Norma that astronomers in the 1980s named the Great Attractor. It sits awkwardly behind the plane of our own galaxy — the Zone of Avoidance, where dust and stars block the view — which is part of why it took so long to find something that large in our own neighbourhood. And the Great Attractor turned out to be a waypoint rather than a destination: beyond it, at around 650 million light years, sits the Shapley Supercluster, the densest concentration of mass in this part of the universe. Much of the pull we attributed to Centaurus is really Shapley showing through.
In 2014 Tully and colleagues mapped these flows and drew a boundary around all the galaxies whose motions converge on the same focus, calling the whole basin Laniakea — Hawaiian for immeasurable heaven. About 500 million light years across, a hundred thousand galaxies. Virgo is one lobe of it; we're out near the edge. It's a watershed definition rather than an object: the set of things falling the same way.
Two complications worth having. The first is that not all of the motion is attraction. In 2017 Hoffman's group showed that roughly half of it is a push — we sit next to a large underdense region, the Dipole Repeller, and emptiness on one side works exactly like mass on the other. We're being shoved as much as pulled. The second is that the fall doesn't complete. Laniakea isn't gravitationally bound; dark energy is winning at this scale. The expansion between us and Shapley is growing faster than the infall can close it, so we will never arrive, and eventually the supercluster will be pulled apart and its members carried off from each other entirely. Tully said as much when he named it.
So the ladder does have a top, and at the top the pattern you started with fails. The Earth's fall toward the Sun never lands but resolves into something stable and repeating — a revolution, as you put it. Laniakea's fall doesn't even get that. It's a leaning-toward that never becomes an orbit and never arrives, and is finally undone by the space opening underneath it. The only fall in the whole stack that goes nowhere at all.
No comments:
Post a Comment